Weed science has many potent plant pest weapons. Yet one powerful tool - basic information about how, where and why different weeds grow in fields - has been in short supply.

University of Nebraska weed scientists are filling that gap with the first comprehensive scientific information about weed distribution, densities and growth patterns in typical crop fields. This research is yielding weed scouting recommendations and other findings to help farmers make better management decisions and control weeds with less herbicide.

"We're encouraging a more information-intensive approach to managing fields so that you know what weeds are out there and where, and you match your weed control to the biology of that population," explained Dave Mortensen, Institute of Agriculture and Natural Resources weed ecologist leading this research.

Since 1991, Mortensen's team has intensively sampled more than 50 fields studying weed distribution and density. Sometimes guided by Global Positioning Systems to specific field sites, they count every weed in a given area. In the lab, researchers create computerized maps by matching weed counts with soils and other site-specific information using Geographic Information System computer software.

"We've learned that, without question, weeds are quite patchy," Mortensen said.

"There are large areas - 30 to 60 percent - of many fields where weed infestations are extremely low."

Farmers know weeds are unevenly distributed in fields, but that patchiness makes accurate estimates difficult.

"They needed a way to more accurately scout fields without hassles," Mortensen said.

The IANR team devised improved scouting techniques that account for weed distribution growth characteristics. Recommendations involve dividing fields into four blocks and scouting five sites per block. This approach more accurately reflects weed variability than scouting field edges or a few spots in a field.

Some farmers sample more intensively. Mortensen agrees more information is better, but the team devised a method practical for most growers.

Scouting 20 field sites shows spatial differences, such as problem spots and weed-free areas. Farmers then can decide whether to treat the field uniformly based on average weed conditions or to site-specifically tailor controls, Mortensen said.

WeedSOFT, NU's weed management software program, includes these improved scouting recommendations. At least 500 farmers, consultants, Cooperative Extension educators and others use WeedSOFT to manage weeds on more than a half million Nebraska crop acres.

"Whether or not they're using WeedSOFT, sampling shows growers the value of getting out in fields and seeing what's going on. It emphasizes the value of keeping better records on fields," he said.

Producers who scout and map fields have more control options. That can mean less herbicide and lower per-acre costs.

"We are often seeing that reduced rates of herbicides will work as well as the full rate when data is collected and weed control is timely," he said.

While field information can be integrated into computerized site-specific farming systems, it doesn't require high-tech tools, Mortensen said.

Flagging and later spot spraying a few weedy acres instead of the whole field is one example of a simple site-specific approach, he said.

Researchers also found weed populations and weed control efficacy are linked.

When weed infestations are low, many different kinds of weed control work well, researchers found. When weeds are thick, controls seldom work.

"The implication is that if things are bad, we need lots of weed-killing power," Mortensen said. "But if they aren't, we can back off considerably from what would be standard recommended control for that field. Most of the fields we've been in are in the category of backing off."

Mortensen also found soil type influences where weeds grow as well as weed control's effectiveness, which he's studying.

The North Central Regional Integrated Pest Management program helps fund this research.

- Vicki Miller

Above: Weed Scientist Dave Mortensen counts every weed within a rectangular sampling grid at one of several sampling sites in a soybean field. Mortensen's team has intensively sampled many fields to learn about weed distribution, densities and growth patterns. Researchers load field information into computers. They use Geographic Information System programs to create detailed weed distribution maps like the one below. White and blue areas have no or few weeds. Yellow and red areas have lots. This research yielded the first comprehensive scientific information about how, where and why weeds grow in fields.